1. Field of the Invention
The invention relates to the field of communication devices. More particularly, the invention relates to communication signal recovery.
2. Description of the Related Art
Communication systems that are subject to frequency shifts and multipath interference may employ pilot signals along with information signals, with the pilot signal largely serving as a reference to aid in the precise demodulation of the information. Mobile devices, in particular, may be subject to frequency and phasing shifts caused by Doppler effects, to fading and multipath interference, and a predictable pilot signal may serve to simplify the interpretation of a received signal in the presence of such effects. Accordingly, pilot signals may accompany primary information signals in many communication systems, especially mobile communications systems, and particularly those using spread spectrum Code Division Multiple Access (CDMA) transmission techniques.
A CDMA system may be designed to support one or more CDMA standards such as (1) the Telecommunications Industry Association (TIA)/Electronic Industries Association (EIA) xe2x80x9cTIA/EIA-95 Mobile Station-Base Station Compatibility Standard for Dual-Mode Wideband Spread Spectrum Cellular Systemxe2x80x9d (the IS-95 standard), (2) the standard offered by a consortium named xe2x80x9c3rd Generation Partnership Projectxe2x80x9d (3GPP) and embodied in a set of documents including Document Nos. 3G TS 25.211, 3G TS 25.212, 3G TS 25.213, and 3G TS 25.214 (the W-CDMA standard), (3) the standard offered by a consortium named xe2x80x9c3rd Generation Partnership Project 2xe2x80x9d (3GPP2) and embodied in a set of documents including xe2x80x9cC.S0002-A Physical Layer Standard for cdma2000 Spread Spectrum Systems,xe2x80x9d the xe2x80x9cC.S0005-A Upper Layer (Layer 3) Signaling Standard for cdma2000 Spread Spectrum Systems,xe2x80x9d and the xe2x80x9cC.S0024 cdma2000 High Rate Packet Data Air Interface Specificationxe2x80x9d (the CDMA 2000 standard), and (4) some other standards. A pilot signal may, for example, serve in each of the described CDMA systems as a phase reference for demodulating a traffic signal or a data signal.
However, when the pilot signal itself is subjected to substantial noise, such as may be caused by fading, multipath interference or loss of signal strength, the ability to precisely determine the pilot signal timing is degraded, impairing its functionality. In conditions where a receiver is moving at a high rate of speed relative to the transmitter, or in conditions of low Signal to Noise Ratio (SNR) the bandwidth and amplitude of the pilot signal can change drastically from nominal conditions. Under high-speed conditions, Doppler effects may cause the pilot signal bandwidth to increase beyond a filter bandwidth, resulting in a loss of part of the signal. The loss of a portion of the pilot signal degrades receiver performance. Thus, in order to enhance the ability of pilot signals to aid in the reconstruction of other information signals, there exists a need for improvements in the ability to resolve pilot signals in the presence of noise and other distortions.
A method and apparatus are disclosed wherein a pilot signal is received and the bandwidth of the pilot signal is estimated, and based on that information the bandwidth of a pilot filter is adjusted. The pilot signal bandwidth may be estimated by comparing a pilot signal power over two or more different frequency ranges. The two or more different frequency ranges over which the power of the pilot signal is evaluated may be established in at least three ways. First, the frequency ranges may overlap each other. For example, the first frequency range, H1, may be a lowpass frequency defined from 0 Hz to a particular cutoff, while the second frequency range, H2, spans 0 Hz to a cutoff frequency exceeding that of H1, so that the frequency range of H2 encompasses that of H1. Second, the frequency ranges may be substantially non-overlapping, for example with H1 ranging from 0 Hz to a first frequency, and H2 defining a frequency band which begins and ends at a frequency higher than the cutoff frequency of H1. Third, the frequency ranges may be established by approximating a Fourier transform of the pilot signal source at two or more frequencies, such as by performing correlations of the pilot signal source with two or more selected signals having different frequencies.
In each case, the magnitude of the signal in the two or more frequency bands is determined, typically by evaluating the signal in terms related to signal power. The noise magnitude, such as power per unit bandwidth, may also be estimated, such as by sampling the pilot signal source over a frequency range which is definitely out-of-band for the pilot signal, and the noise magnitude thus determined may be subtracted from the raw magnitude observed in the two or more frequency bands in order to obtain a better estimate of the magnitude of the pilot signal alone within the two ranges. Then, the ratio of the net signal magnitude in the two or more bands will be used to more accurately select the filter to be applied to the pilot signal source to isolate the pilot signal filter. Evaluating more frequency bands or points may provide a better indication of the appropriate filter bandwidth to use for the pilot signal.